“Slide-ring gels”, new gels different from physical gels and chemical gels, have been developed in recent years. A compound that is used for such slide-ring gels and is drawing attention is a crosslinked polyrotaxane.
A crosslinked polyrotaxane has a structure in which linear molecules are threaded through cyclic molecules in a skewered manner and the cyclic molecules are movable along the linear molecules (has a pulley effect). The pulley effect allows the crosslinked polyrotaxane to be viscoelastic and to uniformly distribute tensile force applied thereto. The crosslinked polyrotaxane is therefore not likely to have cracks or flaws, i.e., has excellent characteristics that conventional crosslinked polymers do not have. Such a crosslinked polyrotaxane is obtainable by placing a capping group at each end of a linear molecule of pseudopolyrotaxanes and to prevent dissociation of the cyclic molecules of pseudopolyrotaxanes, and crosslinking the resulting polyrotaxanes. The pseudopolyrotaxanes have a linear molecule which is included in the cavities of the cyclic molecules in a skewered manner.
For the cyclic molecules of the polyrotaxane, cyclodextrins are favorably used. Cyclodextrins, however, contain a large number of hydroxy groups and these hydroxy groups are firmly bonded to one another by a large hydrogen bonding strength. Therefore, the resulting polyrotaxane is hardly dissolved in water, limiting the application range.
Patent Literature 1 discloses a hydrophilic modified polyrotaxane that is dissolved in water or a water-based solvent, which may extend the application range to coatings, adhesives, and the like. A hydrophilic modified polyrotaxane is typically produced by modifying the hydroxy groups on a cyclodextrin of a polyrotaxane with hydrophilic modifying groups in a solvent, thereby yielding a hydrophilic modified polyrotaxane in an aqueous solution state. This aqueous solution of the hydrophilic modified polyrotaxane may be used as it is, i.e., in a solution state without drying. When a solution of the hydrophilic modified polyrotaxane having a higher concentration than the obtained solution is required, however, a complicated process for concentration is needed. In addition, particularly if the solution of the hydrophilic modified polyrotaxane is to be given another function through a chemical modification and the solution contains water, the water in the solution may inhibit the chemical modification reaction, limiting the application range. To prevent this, Patent Literature 1 discloses a method of producing a solid hydrophilic modified polyrotaxane by freeze-drying of a solution of the hydrophilic modified polyrotaxane.